Coenzyme A, named for its role as an acetyl group carrier,
contains the vitamin pantothenic acid as an essential
constituent (Fig. 10). The synthesis of this vitamin can be
accomplished by green plants, fungi, and most bacteria,
but not by the human body. Its unusual chemical structure
provides the necessary shape and chemical properties to
allow it to bind into crevices within the active sites of a
variety of proteins. There it not only fits snugly but has
electrostatic bonding interactions that allow the proteins
to hold it in just the correct orientation for its function.
Curiously, the exact biological need for the unusual structure
of the vitamin is still obscure. The chemically functional
end of coenzyme A is the sulfhydryl (—SH) group
which is added on to the vitamin structure by cells, as
shown in Fig. 10. Before coenzyme A can function it must
be combined in a thioester linkage with a carboxylic acid
such as acetic acid, or a long-chain fatty acid, as illustrated
in Fig. 10 (right). It is customary in discussions of
metabolism to indicate the bulk of the coenzyme structure
as CoA. The free coenzyme is designated CoA—SH, and
in a thioester the hydrogen of the —SH group is replaced
by an acyl group. The coenzyme has two functions. First,
it can carry the acyl group from one protein to the next in a
metabolic sequence, such as that of Fig. 12. Second, it can
activate a hydrogen atom adjacent to the carbonyl (C=O)
group for removal of a proton (H+) by a catalytic group of
basicnature, suchas —NH2, present intheprotein. Thecarbonyl
group in a thioester is an electron accepting group,
whose facilitation of the proton removal is often indicated
by curved arrows, as shown in Fig. 10 (right). The product
of this proton removal is a reactive anion which is able
to undergo formation or cleavage of carbon–carbon bonds
or dehydrogenation by the riboflavin-containing FAD, as
shown in Fig. 12.

Other coenzymes and prosthetic groups may also act
as acyl group carriers. For the biosynthesis of fatty acids,
a shortened version of coenzyme A (phosphopantetheine,
Fig. 10), is covalently linked to appropriate proteins. During
carbohydrate metabolism, a prosthetic group consisting
of bound lipoic acid (Fig. 11) carries acetyl groups.
Both acetyl groups and long-chain fatty acyl groups are
carried across membranes into and out of mitochondria
while attached to the unusual amino acid carnitine (Fig. 4).
Carnitine is not a vitamin but acts as a coenzyme.